1. Field of the Invention
The present invention relates to a bandpass filter, a duplexer using the filter, a high-frequency module using these, and a communications device using the module, more particularly to a bandpass filter used in the rf stage of a microwave band mobile communications apparatus, a duplexer using the filter, a high-frequency module using these, and a communications device using the module.
2. Description of the Related Art
Recently, mobile communications apparatus, especially portable telephones, are being made small-scale and use higher frequencies, and consequently there is increasing demand for a small-scale and narrow-band bandpass filter and duplexer, used in the rf stage and the like of such apparatus.
FIG. 10 shows an electrode pattern of a conventional combine bandpass filter. In FIG. 10, a bandpass filter 1 comprises a ground electrode 2, microstrip line resonators 3, 4, 5 and 6, which are distributed constant line resonators having length of roughly one quarter of the wavelength of the intended frequency, one end of each being an open terminal and the other end connecting to the ground terminal 2 to form a ground terminal, an input terminal 7 and an output terminal 8. The input terminal 7 and the output terminal 8 are respectively connected to the microstrip line resonators 3 and 6. Then, the above components are provided on part of one main face of, for instance, a printed substrate, which has a ground terminal provided roughly completely over its other main face, so as to form a bandpass filter. Or, as shown in FIG. 11, the above components are provided on a main face of a small dielectric substrate 9, which has a ground terminal provided roughly completely over its other main face, so as to be used as a single chip component. In FIG. 11, like members to FIG. 10 are designated by like reference characters.
In the bandpass filter 1 of the above constitution, a signal input from the input terminal 7 to the microstrip line resonator 3 is input to a filter circuit comprising the microstrip line resonators 3, 4, 5 and 6. The microstrip line resonators 3, 4, 5 and 6 resonate at their intended frequencies, and in addition, they are coupled together by a particularly strong magnetic field generated near their ground terminals, thereby operating as a bandpass filter, allowing only signals close to their intended frequencies to pass and reflecting signals at other frequencies. Then, signals at the intended frequencies are output from the microstrip line resonator 6 to the output terminal 8.
FIG. 12 shows pass characteristics and reflection characteristics of the bandpass filter 1. In FIG. 12, characteristics a represents insertion loss, characteristic b represents reflection loss, and there is a passband of approximately 400 MHz around 4 GHz.
However, as shown in FIG. 12, in the above bandpass filter 1, the insertion loss characteristic a has an attenuation extreme pl only on the high side. Generally, in a bandpass filter, a large amount of attenuation of insertion loss in regions other than the passband is desirable, but in a normal combine filter either, only one such attenuation extreme is formed or absolutely no attenuation extreme is formed, and consequently it is not possible to obtain a sufficient amount of attenuation in the attenuation regions, which include frequency bands on both sides of the passband. More specifically, as shown in FIG. 12, insertion loss of not more than -40 dB (target value AL) at 3.4 GHz is needed on the low side, and insertion loss of not more than -40 dB (target value AH) at 4.6 GHz is needed on the high side, but in fact the insertion loss in each case is only -22 dB and -23 dB respectively.